Virus Capsid Threedimensional Structure

X-ray crystallographic analysis of the Mengo virion at 0.3 nm resolution was reported in 1987 by Luo et al. The overall architecture of the capsid follows T = 1 icosahedral symmetry, with one asymmetric unit comprising one molecule each of VP1, VP2 and VP3. The 60 VP4 molecules occupy internal positions, extending from under the threefold axes to form part of an annular structure under the fivefold axes; there

Table 1 Physicochemical properties of the cardiovirion

Hydrated diameter

30 nm

Thickness of the capsid shell

5 nm

Diameter of the RNA-containing core

20 nm

Sedimentation coefficient (S20>w)

155 x 10"13s"1

Diffusion coefficient (D2o,w)

1.47 x 10~7cm2s~1

Partial specific volume (v)

0.67 ml g 1

Particle weight

8.4 x 10®

Percentage RNA by weight

33%

Virions/A26o unit

9.5 X 1012

are no extensions of VP4, or the other capsid proteins into the RNA-containing core. The shell-forming domain of each of the VP1, 2 and 3 proteins is a wedge-shaped, eight-stranded antiparallel ft barrel. This folding pattern has been called the RNA virus core (RVC) motif because it is shared by the capsid proteins of all picornaviruses and by the T = 3 icosahedral plant viruses. While Nature has adopted a common mode for capsid shell construction for both of these groups of viruses, the specific portions of the polypeptide chains which link the eight /? strands (designated A to H from N- to C-terminal) are different for each virus and for each VP protein type within each virus. It is essentially the composite of these differences that distinguish the various species.

The arrangement of the RVC domains of the capsid polypeptides with respect to the surface of the Mengo virion is shown in Fig. 1. The asymmetric structure unit, or protomer, is outlined by a thick line, as is the pentameric subunit which serves as the assembly unit for the construction of progeny virions in vivo. The protomer is stabilized by extensive noncovalent interactions among the RVC domains of the three polypeptides and between the N-terminal extensions of VP1 and VP3, which are located on the inner surface of the capsid. Protomers can only be dissociated in vitro by boiling in sodium dodecyl sulfate (SDS). The association of protomers into pentamers, which is the first step in progeny virion assembly, is facilitated by hydrophobic contacts between the N-terminal extensions of VP3 and VP4 (actually, VP0 at the time of pentamer formation), which together form a /? annulus in the interior portion of the shell directly under the fivefold symmetry axis. Pentamers can be dissociated into protomers by 2M urea. The 12 pentamers that comprise the cardioviral capsid are held together by electrostatic interactions among the VP2 and VP3 polypeptides along the twofold and around the threefold axes; these can be broken in the presence of 0.14 M chloride or bromide ions at 5.6 < pH < 6.4. Polio-, rhino- or foot-and-mouth disease (FMD)

Fmd Virus Structure

Figure 1 Exterior surface of the Mengo virus capsid. (A) Computer graphics representation of the Mengo virion surface at a resolution of 3.0 A (Luo et at., 1987). Light-colored regions define the higher 'massif regions on the virion surface relative to the lower, dark-colored regions. (B) The surface figure is highlighted with the icosametric arrangement of polypeptides, protomers and pentamers (one pentamer, and the five protomers within it are outlined with thick lines). The icosahedral fivefold, threefold and twofold axes are marked, as are the locations of the 'pits', the putative receptor-binding sites (ovals) and the composite antigenic determinant (squares). The VP4 polypeptides occupy internal locations. Figures A and B, together, form a stereopair for three-dimensional perspective.

Figure 1 Exterior surface of the Mengo virus capsid. (A) Computer graphics representation of the Mengo virion surface at a resolution of 3.0 A (Luo et at., 1987). Light-colored regions define the higher 'massif regions on the virion surface relative to the lower, dark-colored regions. (B) The surface figure is highlighted with the icosametric arrangement of polypeptides, protomers and pentamers (one pentamer, and the five protomers within it are outlined with thick lines). The icosahedral fivefold, threefold and twofold axes are marked, as are the locations of the 'pits', the putative receptor-binding sites (ovals) and the composite antigenic determinant (squares). The VP4 polypeptides occupy internal locations. Figures A and B, together, form a stereopair for three-dimensional perspective.

virions do not share this capsid fragility because their pentamer-pentamer contacts are more extensive: adjacent pentamers in polio- or rhinovirions actually interdigitate and those in FMD virions overlap, while the Mengo pentamers simply abut one another.

Among the noticeable surface features of the Mengo capsid is the regular disposition of 60 depressions, or 'pits', which are about 3 nm in diameter at the top and 2.2 nm deep. In accordance with Rossmann's 'canyon hypothesis', indirect experimental evidence implicates the 'pits' and the adjacent walls of the nearby fivefold 'massifs' as virion-attachment or receptor-binding sites. The outer diameter of the 'pit' is small enough to prohibit access by an antibody molecule (whose combining-site 'footprint* is about 3.5 nm in diameter), yet large enough to accommodate an elongated cell surface glycoprotein. Mutations changing amino acid residues around the 'pit' would be permitted, whereas the residues which line the 'pit' would have to be conserved in order for virus—cell interaction to occur. The immunoglobulin VCAM has been proposed as a cell surface receptor for the EMC-like cardioviruses; however, EMC and Mengo are also very efficient at agglutinating erythrocytes, and the specific receptor for this function on human cells is the sialoglyco-protein, glycophorin A. The sialic acid moiety of glycophorin A is specifically required for erythrocyte binding.

Figure 1 also shows the location of the composite antigenic determinant defined by a panel of neutralizing monoclonal antibodies. Although each antibody recognizes a specific epitope within the determinant

(generally a conformational rather than a linear array of amino acid residues), crossneutralization experiments have shown that most epitopes are physically overlapping. The determinant involves residues in VP1 (T1100; in the conventional numbering system the first digit refers to the capsid polypeptide and the next three identify its position in the chain from the N-terminus), VP2 (K2075, N2144, R2145, S2147 and K1248) and VP3 (K3057 and S3068). These residues were identified by sequencing the capsid-coding regions of RNAs from neutralization-escape mutants. As expected, all of these residues are on the virion surface, generally near the outer wall of the 'massif' at the center of each asymmetric protomer, and spatially removed from the 'pit' areas.

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